Drugs that are administered in the form of conventional tablets or capsules become available to body fluids at a rate that is initially very high, followed by a rapid decline. For many drugs, this delivery pattern results in a transient overdose, followed by a long period of underdosing. This is a pattern of limited clinical usefulness. Improved delivery patterns were first made available in the 1970's with the introduction of a variety of controlled delivery systems. These systems lowered the amount of drug released immediately after dosing and extended the time period over which drug release continued, thereby minimizing both the overdose and the underdose effects. These improvements provided effective medication with reduced side effects, and achieved these results with reduced dosing frequency.
Many of these controlled delivery systems utilize hydrophilic, polymeric matrices that provide useful levels of control to the delivery of drugs. Such matrices do not provide adequate control over the drug release rate, but instead provide a release pattern that approximates square-root-of-time kinetics in which the total amount of drug released is approximately proportional to the square root of the elapsed time. With this release pattern in an aqueous medium, much of the drug in the matrix of many of these formulations is released into an aqueous medium within the first hour.
The benefits of a constant release rate with regard to prolonging therapeutic efficacy while minimizing side effects are well established. It is well known in the art that a nearly constant release rate that simulates zero order kinetics can be obtained by surrounding a tablet core with a membrane or coating. The membranes or coatings described in the art are typically 1-5% of the weight of the tablet. Unfortunately, swelling of the tablet can disrupt the membrane and change the kinetics considerably from zero order. U.S. Pat. No. 4,892,742, issued Jan. 9, 1990 (assignee: Hoffman-La Roche Inc.; inventor: Shah) discloses a tablet consisting of:                1) a core consisting of 5-35% of a water insoluble polymer matrix and 65-95% of a water soluble active ingredient; and        2) a membrane coating comprising 5-10% of the weight of the tablet and consisting of a rate-controlling polymer.        
The preferred coating material is ethyl cellulose or a plasticized ethyl cellulose and is a typical controlled release coating for a tablet. The lack of swelling of these membranes and the insoluble core allow the membrane coating to remain intact throughout the release process without breakage, thereby preventing exposure of the core. Without swelling to a minimal size, neither gastric retention of the tablet nor sustained delivery of the active ingredient to the upper gastrointestinal (GI) tract would be achieved.
U.S. Pat. No. 4,629,620, issued Dec. 16, 1986 (assignee: AB Ferrosan; inventor: Lindahl), describes membrane-coated sustained-release tablets where the membrane is an insoluble polymer containing pore-forming agents. Like the tablets and membrane coatings of the Shah patent (no. 4,892,742), the tablets and membranes of the Lindahl patent are non-swelling and are not retained in the upper GI tract.
U.S. Pat. No. 5,500,227, issued Mar. 19, 1996 (assignee: Euro-Celtique, S.A.; inventor: Oshlack) discloses the use of a controlled release tablet that consists of:                1) an immediate release tablet core containing an insoluble drug; and        2) a thin hydrophobic coating material.        
This patent does not include any disclosure or suggestion that either the membrane or the tablet swells, and thus the patent does not disclose a manner of confining controlled release to the upper GI tract.
U.S. Pat. No. 4,756,911, issued Jul. 12, 1988 (assignee: E.R. Squibb & Sons, Inc.; inventor: Drost) discloses a controlled release tablet for procainamide hydrochloride consisting of:                1) a core containing about 70% (on a weight basis) of the drug, from 5 to 15% by weight of the hydrocolloid gelling agent, hydroxypropylmethyl cellulose, and from 0 to 8% of non-swellable binders; and        2) a water permeable coating film comprised of a mixture of at least one hydrophobic and one hydrophilic polymer.        
This patent teaches that the entry of water through the film coating causes the membrane to peel off in 2 to 4 hours after ingestion of the tablet. Drug release proceeds from the core alone.
U.S. Pat. No. 4,891,223, issued Jan. 2, 1990 (assignee: Air Products and Chemicals, Inc.; inventor: Ambegaonkar) discloses compositions containing:                1) an active ingredient that is soluble in the release medium;        2) an inner coating that is water soluble and swellable; and        3) a second outer coating that is water insoluble.        
The second outer coating is disclosed as being able to stretch sufficiently to remain in contact with the inner layer, but the second outer coating still may limit the swelling of the composition. The invention described involves controlled-release beads rather than tablets and are far below the size that is necessary to confine release of the active ingredient to the upper GI tract.
The prior art also includes disclosures of multilayer tablets designed to provide release profiles that are intermediate between square-root-of time and zero-order. This prior art is listed below. The multi-layered tablets disclosed in the these patents may swell sufficiently to allow controlled delivery to the upper GI tract, but they do not include a swelling outer layer that fully encloses a core. The outer layers are only partial, discontinuous coatings and thus are not subjected to the large strains that are caused by differential swelling.
U.S. Pat. No. 5,783,212, issued Jul. 21, 1998 (assignee: Temple University; inventor: Fassihi) discloses a three-layer tablet, i.e., a core with a partial coating on only two sides, described as:                1) a drug layer consisting of a swellable, erodible polymer; and        2) two barrier layers comprising swellable, erodible polymers that erode and swell faster than the drug layer.        
There is no disclosure or suggestion that the swelling and erosion are matched among the three layers, nor is there any recognition that the drug layer swells faster. There is no disclosure of a swelling membrane or any recognition of the loss of control over the release rate caused by a disrupted membrane.
U.S. Pat. No. 5,549,913, issued Aug. 27, 1996 (assignee: Inverni Della Beffa, S.p.A.; inventor: Colombo), teaches the use of a three-layer tablet where:                1) two external layers, each covering only one side, comprised of hydrophilic swelling polymers and at least one of which contains drug; and        2) an interposing layer controlling the release of the drug.        
In this multilayer tablet, the drug is released not through a swelling membrane or coating, but instead through an erodible or soluble layer.
Conte et al., in Biomaterials 17 (1996):889-896, disclose two- and three-layer tablets with barrier layers that swell or erode. These barrier layers are described as partial coatings and as such do not form barriers that must remain intact under the pressure arising from cores surrounded by coatings that swell at different rates.
Published international application WO 99/47128, published Sep. 23, 1999 (applicant: Bristol-Myers Squibb; inventor: Timmins) discloses a pharmaceutical tablet consisting of:                1) an inner phase containing drug and an extended release material; and        2) an outer phase that is continuous and comprised of an extended release material;the inner phase being dispersed throughout the outer phase. The extended release materials described in WO 99/47128 can swell substantially to confine delivery to the upper GI tract. The outer continuous phase is a dispersion and not a coating or membrane. The drug release profiles resulting from this invention consequently deviate substantially from zero-order and actually exhibit a release profile that is proportional to the square root of time.        
One method of prolonging the release of a highly water-soluble drug is disclosed in International Patent Application Publication No. WO 96/26718, published Sep. 6, 1996 (applicant: Temple University; inventor: Kim). The method disclosed in WO 96/26718 is the incorporation of the drug into a polymeric matrix to form a tablet that is administered orally. The polymer is water-swellable yet erodible in gastric fluids, and the polymer and the proportion of drug to polymer are chosen such that:                (i) the rate at which the polymer swells is equal to the rate at which the polymer erodes, so that the swelling of the polymer is continuously held in check by the erosion, and zero-order release kinetics (constant delivery rate) of the drug from the matrix are maintained;        (ii) the release of drug from the matrix is sustained over the full erosion period of the polymer, the tablet therefore reaching complete dissolution at the same time that the last of the drug is released; and        (iii) release of the drug from the matrix is extended over a period of 24 hours.        
A key disclosure in WO 96/26718 is that to achieve the release of drug in this manner, the polymeric matrix must be a polymer of low molecular weight. If, by contrast, a polymer of high molecular weight is used and the swelling rate substantially exceeds the erosion rate, the lack of erosion will prolong even further the delivery of the drug residing close to the center of the tablet and even prevent it from being released. Thus, there is no disclosure in WO 96/26718 that a drug of high water solubility can be released from a high molecular weight polymer in a period of time substantially less than 24 hours, or that any advantage can be obtained by the use of a polymer that does not erode as quickly as it swells. This is particularly significant since any tablet, including swollen tablets, will pass from the stomach after the termination of the fed mode, which typically lasts for only 4 to 6 hours. Moreover, this patent does not teach the use of a membrane or coating, much less one that swells and stays in contact with the core throughout the release of the drug.
In many cases, the passage of a drug from the stomach into the small intestine while the drug is still in a tablet or other dosage form raises problems that lower the therapeutic efficacy of the drug, due to either the absence of the favorable conditions in the stomach, the exposure to unfavorable conditions in the colon, or both.
For example, most orally administered antibiotics are capable of altering the normal flora of the gastrointestinal tract, and particularly the flora of the colon. One result of these alterations is the overgrowth of the organism Clostridium difficile, which is a serious adverse event since this organism releases dangerous toxins. These toxins can cause pseudomembranous colitis, a condition that has been reported as a side effect of the use of many antibiotics due to passage of the antibiotics from the stomach through the GI tract to the small intestine. In its milder forms pseudomembranous colitis can cause mild nausea and diarrhea, while in its stronger forms it can be life-threatening or fatal. Examples of antibiotics that pose this type of threat are amoxicillin, cefuroxime axetil, and clindamycin. Cefuroxime axetil (i.e., the axetil ester of cefuroxime), for example, becomes active when hydrolyzed to free cefuroxime, but when this occurs prior to absorption, damage to essential bacterial flora can occur. Hydrolysis to the active form typically occurs in the tissues into which the ester has been absorbed, but if the ester reaches the lower intestine, enzymes in the lower intestine cause the hydrolysis to occur in the intestine itself, which not only renders the drug unabsorbable but also converts the drug to the active form where its activity alters the flora. Further examples are clarithromycin, azithromycin, ceftazidime, ciprofloxacin, and cefaclor. A goal of the present invention is to avoid antibiotic-induced overgrowth of the lower intestinal flora by administering antibiotics, regardless of their level of solubility, in a manner that confines their delivery to the stomach and upper small intestine.
A class of drugs that suffer a loss of benefit from rapid initial release are those that are susceptible to degradation by exposure to gastric fluid, either due to the action of gastric enzymes or as the result of low solution pH. One example of such a drug is topiramate, a drug that is used for the treatment of epilepsy. Topiramate is absorbed most rapidly in the upper GI tract, but when made available at this site, it is hydrolyzed by the acidic environment of the stomach. Avoidance of this high rate of hydrolysis requires a dosage form that does not expose the drug to the acidic environment for an extended period.
A class of drugs that suffer a loss of benefit when allowed to pass into the small intestine are those that are absorbed only in the upper GI tract and suffer from incomplete absorption or from wide differences in absorption, both within a single patient and between different patients. One example of such a drug is cyclosporine, a drug of low solubility that is used as an immunosuppressant to reduce organ rejection in transplant surgery. In addition to its low solubility, cyclosporine has a low absorption rate of about 30% on average, together with wide absorption variability ranging from as little as 5% in some patients to as much as 98% in others. The variability is attributable in part to differences among the various disease states existing in the patients to whom the drug is administered, and in part to differences in the length of time between the transplant surgery and the administration of the drug. The variability can also be attributed to the poor aqueous solubility of the drug, variations in the gastric emptying, variations in the length of time required for intestinal transit between the stomach and the colon, variations in mesenteric and hepatic blood flow, variations in lymph flow, variations in intestinal secretion and fluid volume, variations in bile secretion and flow, and variations in epithelial cell turnover.
Another class of drugs that suffer a loss of benefit when allowed to pass into the small intestine are drugs that are susceptible to degradation by intestinal enzymes. The degradation occurs before the drug can be absorbed through the intestinal wall, leaving only a fraction of the administered dose available for the intended therapeutic action. An example of such a drug is the pro-drug doxifluridine (5′-deoxy-5-fluouridine (dFUR)). The activity of this pro-drug depends on its activation to 5-fluorouracil by pyrimidine nucleoside phosphorylases. These enzymes are found in tumors as well as in normal tissues, and their activity in tumor cells is more than twice their activity in normal tissue. In addition, these enzymes demonstrate their highest activity in the large intestine. When doxifluridine is administered orally, it risks being converted to 5-fluorouracil in the intestine before it reaches the tumors. 5-Fluorouracil is much more toxic than doxifluridine and causes intestinal toxicity (nausea and diarrhea) and severe damage to the intestinal villi. Other drugs that can produce a similar effect upon reaching the colon are cyclosporine and digoxin.
A further class of drugs whose effectiveness declines when the drugs are allowed to pass into the large intestine are those that are susceptible to inactivation by drug transporters that reside in lower gastrointestinal tract enterocytes. The inactivation occurs before the drug penetrates the intestinal wall, leaving only a fraction of the administered dose available for the intended therapeutic action. One example of a drug transporter is the p-glycoprotein efflux system, in which a p-glycoprotein acts as an absorption barrier to certain drugs that are substrates for the p-glycoprotein. The barrier acts by attaching to these drugs and transporting them drug back into the lumen, e.g., the duodenum, jejunum/ileum or colon, from which they were absorbed, or by preventing them from being absorbed at all. This restriction of the drug to the interior of the GI tract is effectively an inactivation of the drug if the drug must pass out of the GI tract into the bloodstream to be effective. Thus, while the p-glycoprotein efflux system is useful in many respects, such as preventing toxic compounds from entering the brain, it interferes with the efficacy of certain drugs whose absorption is necessary in achieving the therapeutic effect. The p-glycoprotein concentration is lowest in the stomach and increases in concentration down the GI tract to the colon where the p-glycoprotein is most prevalent. These drugs therefore would benefit from controlled release over an extended period into the upper GI tract where p-glycoprotein is lowest. Cyclosporine is an example of a drug of low solubility that is susceptible to inactivation by the p-glycoprotein efflux system, in addition to its susceptibility to degradation by colonic bacterial enzymes. Other examples of drugs that are susceptible to the p-glycoprotein efflux system are the anti-cancer drug paclitaxel, ciprofloxacin, and the HIV protease inhibitors saquinavir, ritonavir, and rietfinavir.
A still further class of drugs that suffer from loss of effectiveness when not fully absorbed before reaching the colon are drugs that require an acidic environment for effective bioavailability. For certain drugs, the pH at a given site within the GI tract is an essential determinant of the bioavailability of the drug, since the solubility of the drug varies with pH. The stomach has a low pH and thus creates an acidic environment, while the small intestine has a higher pH, creating a slightly acidic to alkaline environment. Some drugs achieve bioavailability only when ionized by the acidic environment of the stomach. Other drugs are more bioavailable in a non-ionized state. Acidic drugs that have a low pK, for example, are in the neutral form in the stomach, and those that are more bioavailable in this state are preferentially absorbed in the stomach or upper duodenum. Examples of highly soluble drugs that meet this description are esters of ampicillin. Examples of low solubility drugs that behave similarly are iron salts, digoxin, ketoconazole, fluconazole, griseofulvin, itraconazole, and micoconazole. Iron salts are used in the treatment of the various forms of anemia, digoxin is used in the treatment of heart disease, and ketoconazole is used in the treatment of systemic fungal infections such as candidiasis, canduria, blastomycosis, coccidiomycosis, histoplasmosis, chronomycosis, and pacococcidiornycosis. Still further drugs that are more absorbable in the neutral form that is maintained at low pH are those whose molecular structure contains at least one group that becomes ionized in the pH range of 5 through 8, which is the pH range encountered in the small intestine and the region of the colonic junction. In addition, zwitterionic drugs may be better alisorbed in a charged form that is present in the acidic environment of the stomach or the duodenal cap. The bioavailability of all of these drugs can be maximized by confining them to the acidic environment of the stomach while controlling their release rate to achieve an extended release profile.
A still further example of drugs that lose their efficacy upon reaching the lower portions of the GI tract are drugs that are soluble in an acidic environment but insoluble in an alkaline or neutral environment. The HIV protease inhibitor nelfinavir mesylate is one example of such a drug. Portions of the drug that are undissolved cannot be absorbed. Portions that are dissolved but not yet absorbed when they pass from the stomach into the small intestine may undergo precipitation and loss of their therapeutic benefit. This is confirmed by the fact that the presence of food in the GI tract substantially increases the absorption of orally administered nelfinavir. Peak plasma concentration and area under the plasma concentration-time curve of nelfinavir are two to three times greater when doses are administered with or following a meal. This is believed to be due at least in part to enhanced retention of the drug in the stomach.